1. Field of the Invention
The present invention relates to a semiconductor device which has a circuit composed of a thin film transistor (hereinafter referred to as TFT), and to a method of manufacturing the semiconductor device. For instance, the present invention relates to an electro-optical device represented by a liquid crystal display panel, and to electronic equipment which has such electro-optical device as its component.
In this specification, the term semiconductor device refers to a device in general that utilizes semiconductor characteristics to function, and electro-optical devices, light emitting devices, semiconductor circuits, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
In recent years, development of a semiconductor device has been advanced, which has a large area integrated circuit composed of a thin film transistor (TFT) that is formed from a semiconductor thin film (several nm to several hundreds nm in thickness) on a substrate with an insulating surface. Typical examples of the semiconductor device include a liquid crystal display device with a liquid crystal module, an EL module, and a non-magnification image sensor.
Of liquid crystal display devices, one that is attracting attention is an active matrix liquid crystal display device in which pixel electrodes are arranged so as to form a matrix and TFTs are used for switching elements that are respectively connected to the pixel electrodes in order to obtain a high quality image.
In a liquid crystal module to be mounted to a liquid crystal display device, a pixel portion for displaying an image and a driving circuit for controlling the pixel portion are formed in different functional blocks on the same substrate. The driving circuit is based on a CMOS circuit and includes a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like.
The pixel portion of the liquid crystal module has several ten thousands to several millions of pixels and each of the pixels is provided with a TFT (pixel TFT). A pixel electrode is provided for each pixel TFT. An opposite electrode is formed on an opposite substrate that faces the substrate which has the pixel portion and driving circuit across the liquid crystal. A kind of capacitor is formed with the liquid crystal as dielectric. The voltage applied to each pixel is controlled by switching function of the TFT, and electric charges given to the capacitor are controlled to drive the liquid crystal and control the amount of light transmitted for displaying an image.
Conventionally, a TFT is formed from an amorphous silicon film. In order to achieve higher performance, it has been attempted to use a crystalline silicon film (typically called a polysilicon film) for an active layer of a TFT (hereinafter referred to as polysilicon TFT). A polysilicon TFT is high in field effect mobility and therefore can make a circuit which has various functions.
A technique for employing such crystalline silicon film over a glass substrate is disclosed in Japanese Patent Application Laid-open No. Hei 8-78329. According to the technique described in the publication, an amorphous silicon film is selectively doped with a metal element that accelerates crystallization (typically nickel) and then subjected to heat treatment to grow a crystalline silicon film from the doped region. Crystal grains obtained by this technique are very large in size.
Owing to the effect of the metal element, the technique is capable of lowering the temperature, at which the amorphous silicon film is crystallized, by 50 to 100° C. as compared to the case where the metal element is not used in crystallization. In addition, time required to complete crystallization is ⅕ to 1/10 of the case where the metal element is not used for crystallization. This technique is superior also in terms of productivity.
The crystalline silicon film obtained in accordance with the technique of the above publication (Japanese Patent Application Laid-open No. Hei 8-78329) has a unique crystal structure. A large number of columnar crystal masses (also called domains) are formed in the crystal silicon film and crystals in one crystal mass (domain) all have the same crystal orientation. The size of one crystal mass (domain) is as large as 200 to 300 μm. Adjacent crystal masses (domains) have different orientations and there is a boundary between adjacent crystal masses. When a TFT is formed so that its channel formation region is confined within one crystal mass, it is expected that almost the same level of electric characteristics as those of single crystal are obtained. However, with the technique of the above publication, crystal masses are formed at random and it is difficult to manufacture a TFT with its channel formation region placed within one of the randomly-formed crystal masses. Accordingly, it is also difficult to form a channel formation region of every TFT in the pixel portion within one crystal mass.
When a crystalline silicon film obtained by the technique of the above publication is used for an active layer of a TFT, the TFT is advantageous in excellent electric characteristics. On the other hand, there is a slight difference, namely, fluctuation, in TFT characteristics between TFTs obtained by the technique due to the presence or absence of a boundary between adjacent crystal masses (between crystal masses with different orientations), or due to varying sizes of crystal masses formed.
If there is a fluctuation in electric characteristics among the TFTs placed in the pixel portion, voltages applied to the respective pixel electrodes are fluctuated and the amount of light transmitted is then fluctuated, to result in uneven display to the eyes of an observer. Although the fluctuation is in an acceptable range and does not cause a problem at present, it is conceivable that the fluctuation is a very serious problem as the pixel size is more minute to obtain an image with higher definition in future.
With the reduction in size of a channel formation region (channel length and channel width) as wall as the upcoming reduction in the width of a wiring line, it is unavoidable that a TFT has its channel formation region at a boundary between crystal masses. TFT characteristics (the mobility, S value, ON current value, and OFF current value, and the like) of the TFT is different from TFT characteristics of TFTs whose channel formation regions are not formed in boundaries, which is considered as a cause of display fluctuation.
Despite several attempts, there have not been found the optimum measure to form a crystalline silicon film which has a uniform grain size at a process temperature equal to or lower than the distortion point of glass substrate, namely, 600° C.
It is difficult to obtain a highly uniform crystalline silicon film and high mobility at the same time in prior art. It is also difficult to manufacture a TFT at a process temperature equal to or lower than 600° C. in prior art.
An active matrix type light emitting device which has an OLED as a self-luminous element (hereinafter simply referred to as light emitting device) is being researched actively. A light emitting device is also called as an organic EL display (OELD) or an organic light emitting diode (OLED).
An OLED, which is self-luminous, does not need back light which is necessary in liquid crystal display devices (LCDs) and is therefore suitable for making a thinner device. In addition, a self-luminous OLED has high visibility and no limitation in terms of viewing angle. These are the reasons why light emitting devices using OLEDs are attracting attention as display devices that replace CRTs and LCDs.
Known as one mode of light emitting devices using OLEDs is an active matrix driving method in which each of pixels has a plurality of TFTs and video signals are sequentially written in the pixels to display an image. In the light emitting device using OLEDs, a TFT is an indispensable element when the active matrix driving method is employed. However, TFTs formed from polysilicon easily fluctuate in characteristics due to defects in grain boundaries.
In light emitting devices using OLEDs, each pixel is provided with at least a TFT for functioning as a switching element and a TFT for supplying current to an OLED. Irrespective of circuit structures and driving methods of pixels, the luminance of a pixel is determined by the ON current (Ion) of a TFT electrically connected to an OLED to supply a current to the OLED. Therefore, when the entire screen displays white, for example, the luminance is fluctuated unless the ON current of each pixel is constant.